1. The Technical Field
The present invention relates to methods for continuous casting of steel slabs.
2. The Prior Art
In conventional continuous casting mills with direct hot charging, steel in a caster assembly is cast into a continuous strand, and passes through a strand containment apparatus in which the steel surface is cooled and the strand changes direction from the vertical to the horizontal. The strand is then conveyed to a severing apparatus where it is severed into slabs, blooms, billets or other products. The slab or other product then enters a reheat furnace for heating to a uniform temperature suitable for downstream rolling and other processing.
It has been widely recognized that it is advantageous to directly charge slabs coming from the caster into the reheat furnace in order to reduce the energy cost associated with reheating slabs that have been cooled to ambient temperature.
In general, problems encountered with plate steel product produced by such continuous casting mills include the tendency for areas around one or more surfaces of the steel product to exhibit brittleness, cracking, sponging, and other surface defects. Surface defects are especially prevalent after the interim steel product is subjected to downstream rolling or other stresses. Although the causes of such surface defects are not completely understood, it has been observed that surface defects tend to occur frequently in steel products having surfaces that are at or above the steel's austenite-to-ferrite transformation start temperature when the product exits the caster assembly, cool to a temperature above the steel's austenite-to-ferrite transformation completion temperature as the product enters the reheat furnace, then are reheated to a temperature above the transformation start temperature when the product is inside the reheat furnace. Steel products that tend to be particularly susceptible to surface defects include low- to high-carbon steels and low-alloy steels, all of which may contain aluminum (Al) and residual elements such as sulphur (S), phosphorus (P), nitrogen (N), and copper (Cu).
While an understanding of the causes of the surface defects is not per se necessary for the practice of the invention, some discussion of the applicant's understanding of the phenomenon may be helpful to the reader. Steel product exiting the caster assembly has a coarse austenite grain structure. As the steel product cools to a temperature above the transformation completion temperature of the metal, various elements including residual elements migrate to the austenite grain boundaries where they will reside as solute elements, or eventually combine to form precipitates. If the steel product has not cooled to below the transformation completion temperature before reheating in the reheat furnace, these elements, in either solute or precipitate form, remain at or near the original austenite grain boundaries. The presence of these elements on grain boundaries and/or the development of precipitate-free zones adjacent to grain boundaries can be detrimental to the ductility of the steel product and may also contribute to the manifestation of one or more types of surface defects. It appears that the principal culprit in many cases is the copper present.
If the interim steel product is taken off-line and left for several hours to cool slowly in still air, the entire product will have completely transformed from coarse-grained austenite to other microconstituents, such as ferrite or pearlite. Reheating this product in a reheat furnace to above the transformation start temperature, (about 900 C. for most steels of interest) the critical temperature above which there is austenite recrystallization, re-transforms the product into fine-grained austenite. It has been found that a product having such a fine-grained austenitic microstructure tends to be free from surface defects. However, such slow cooling requires the product to be taken off-line for an undesirably lengthy period of time, thereby slowing down steel production.
It has been found that instead of re-transforming the entire steel product into fine-grained austenite, it is necessary to re-transform only the surface layers to a suitable depth to achieve a product that is for the most part free of surface defects. However, off-line slow air cooling to achieve a re-transformed layer of sufficient depth requires an undesirably lengthy time.
Previously known methods have been devised in which a slab is taken off-line, immersion-quenched in a quench tank, then returned on-line for transfer into the reheat furnace. In such methods, the temperature of the slab surfaces is often reduced below the transformation completion temperature, i.e. the steel's transformation completion temperature, before the slab is reheated in the reheat furnace. It has been found that an immersion-quenched slab tends to exhibit undesirably inconsistent metallurgical properties along its length. This inconsistency appears to be due to the formation of a lengthwise temperature gradient on the slab prior to its immersion; since the slab is cast from a continuous caster, its downstream portions have had more time to cool than its upstream portions.
One way to reduce the surface temperature, is to quench the surface of the slab with water as it exits the caster. Quenching processes are disclosed in such prior art references as U.S. Pat. No. 5,915,457; U.S. Pat. No. 6,374,901 B1; and U.S. Pat. No. 6,557,622 B2, the complete disclosures of each of which are hereby expressly incorporated herein by reference. The latter two references in particular being directed to methods for differential quenching of the surface of the slabs to address the transverse temperature profile of the slab surfaces.
However, there is a disadvantage to water quenching, in that the casting speed must be restricted to ensure a sufficient depth of the slab has been cooled below a critical temperature, which can negatively impact productivity. Furthermore, certain grades of steel, are susceptible to cracking as a result of the quench, notably vanadium-bearing steels.
It would be desirable to provide a continuous casting process which enjoys the benefits of hot-charging the cut slabs into the reheat furnace to save on energy costs, while avoiding the risks of surface cracking that are associated with water quenching.
This and other desirable characteristics of the invention will become apparent in view of the present specification, including claims, and drawings.